Perpendicularly magnetized MTJs (p-MTJs) are a major emerging technology for use as embedded magnetic random access memory (MRAM) applications, and standalone MRAM applications. P-MTJ MRAM technology using spin-torque (STT-MRAM) for writing of memory bits was described by J. C. Slonczewski in “Current driven excitation of magnetic multilayers”, J. Magn. Magn. Mater. V 159, L1-L7 (1996), and is highly competitive with existing semiconductor memory technologies such as SRAM, DRAM, and flash.
Both MRAM and STT-MRAM have a MTJ element based on a tunneling magnetoresistance (TMR) effect wherein a MTJ stack of layers has a configuration in which two ferromagnetic layers are separated by a thin insulating tunnel barrier layer. One of the ferromagnetic layers called the pinned layer has a magnetic moment that is fixed in an out-of-plane direction such as the +z direction when the plane of each layer is laid out in the x-axis and y-axis directions. The second ferromagnetic layer has an out-of-plane magnetization direction that is free to rotate to either the +z (parallel or P state) or the −z (antiparallel or AP state). The difference in resistance between the P state (Rp) and AP state (Rap) is characterized by the equation (Rap−Rp)/Rp that is also known as DRR. It is important for MTJ devices to have a large DRR value, preferably higher than 1, as DRR is directly related to the read margin for the memory bit, or how easy it is to differentiate between the P state and AP state (0 or 1 bits).
When the free layer has a magnetization direction perpendicular to the plane of the film, the critical current (Ic) needed to switch the magnetic element is directly proportional to the perpendicular anisotropy field as indicated in Equation (1) where e is the electron charge, α is a Gilbert damping constant, Ms is the saturation magnetization of the free layer, h is the reduced Plank's constant, g is the gyromagnetic ratio, and Hkeff,⊥ is the out-of-plane anisotropy field of the magnetic region to switch, and V is the volume of the free layer:
                              i          c                =                              α            ⁢                                                  ⁢                          eMsVH                                                k                  eff                                ,                ⊥                                                          g            ⁢                                                  ⁢            ℏ                                              Eq        .                                  ⁢                  (          1          )                    
The value Δ=kV/kBT is a measure of the thermal stability of the magnetic element where kV is also known as Eb or the energy barrier between the two magnetic states (P and AP), kB is the Boltzmann constant and T is the temperature. For functional MRAM products, the free layer (information storage layer) must have a high enough Eb to resist switching due to thermal and magnetic environmental fluctuations. This energy barrier to random switching is related to the strength of the perpendicular magnetic anisotropy (PMA) of the free layer. One practical way to obtain strong PMA is through interfacial PMA at an interface between a CoFeB free layer and an MgO tunnel barrier layer. Even higher PMA is achieved by forming a second MgO interface for additional interfacial PMA on an opposite side of the free layer with respect to the tunnel barrier. Therefore, total PMA in the free layer is optimized with an MgO/CoFeB/MgO stack in the p-MTJ thereby increasing Eb.
FIG. 1 depicts a conventional p-MTJ 1 wherein an optional seed layer 11, pinned layer 12, tunnel barrier 13, free layer 14, metal oxide cap layer 17, and hard mask 16 are sequentially formed on a substrate that is a bottom electrode 10 in a MRAM structure, for example. Unfortunately, a consequence of employing an MgO cap layer is the addition of parasitic resistance to the p-MTJ device. Equation (2) shows the effect of the cap layer resistance contribution to total MTJ resistance while Equation (3) indicates the impact on DRR.
                              DRR          =                                                                                          R                    AP                                    -                                      R                    p                                                                    R                  p                                            ⁢                                                          ⁢              where              ⁢                                                          ⁢                              R                AP                                      =                                          R                AP                barrier                            +                                                R                  AP                  cap                                ⁢                                                                  ⁢                and                                                    ⁢                                  ⁢                              R            P                    =                                    R              P              barrier                        +                          R              P              cap                                      ⁢                                  ⁢                              Since            ⁢                                                  ⁢                          R              AP              cap                                =                      R            P            cap                                              Eq        .                                  ⁢                  (          2          )                                        DRR        =                                                            R                AP                barrier                            +                              R                AP                cap                            -                              (                                                      R                    P                    barrier                                    +                                      R                    P                    cap                                                  )                                                                    R                P                barrier                            +                              R                P                cap                                              =                                                    R                AP                barrier                            -                              R                P                barrier                                                                    R                P                barrier                            +                              R                P                cap                                                                        Eq        .                                  ⁢                  (          3          )                    
In summary, the series resistance caused by the metal oxide cap layer (RAPcap and RPcap) will cause a reduction in DRR, effectively reducing the MRAM bit reading margin, as well as increasing the bit's writing voltage by adding a series resistance. Since an MgO cap layer or the like is required to achieve strong PMA for enhanced thermal stability, an improved p-MTJ structure is needed such that strong PMA is maintained while significantly reducing the series resistance contribution from the cap layer.